This invention is related generally to devices which convert the energy of wind currents to more usable forms and is more particularly concerned with such devices which are capable of such conversion at high efficiencies and at low cost.
Recently, the world has been confronted with the fact that with rapidly advancing industralization and technology the fossil fuel energy sources in the world are being depleted at an increasingly rapid rate. As a result, renewed emphasis is being placed on efforts to derive usable quantities of energy from other sources, such as nuclear fuels and solar energy.
A fluid current, such as an air current or wind, possesses kinetic energy in a quantity which is a function of its mass and velocity. If the current is deflected from its original path, the object causing the deflection is subjected to a force which is a function of numerous factors. For the purpose of this discussion, the most important of these factors are the area of the surface causing the deflection, and the severity or degree of deflection it causes.
Basically, all conventional windmills employ this principle through aerodynamic reactions which cause pressure differentials across the surfaces of their airfoil blades. However, as stated above, the force available for conversion to other energy forms is a function of the area of the deflecting surface and the degree of deflection. The degree of deflection may be roughly equated to the efficiency of the windmill. This eventually reaches a maximum value. Thereafter, any attempt to further increase the power recovering capability of the windmill requires either an increase in the area of its airfoil surfaces, or an increase in the kinetic energy of the air current.
If airfoil area is to be increased, the blades may be widened to a certain extent; but, eventually, the blades must also be lengthened to increase the frontal area of the windmill and to gain exposure to the kinetic energy of a larger mass of air in the current. Unfortunately, as the length of a rotating blade increases radially, the blade becomes increasingly subject to a variety of structural, dynamic, and operational problems.
For a given rotational speed, the linear speed at the blade tip may become excessive for efficient aerodynamic reaction. This may be corrected by reducing the working rotational speed; however, this produces two additional problems. The rate of input of work to the windmill shaft is reduced, and, due to the disparity in linear velocity over the long span of the blade, the root area may now be operating in or near an aerodynamically stalled condition. Furthermore, since most power converting devices which the windmill might drive require relatively high cranking speeds, some form of step-up gearing must be employed thereby, adding friction, weight and complexity to the windmill system and further reducing its efficiency.
Structurally, if a blade is to be of increased length, yet be aerodynamically practical, it must present large, effectively planar surfaces. Winds of higher than average velocity represent the most productive environment, but such winds acting upon the blade configuration just described are likely to create intolerable stress concentrations, bending, and dynamic flutter. Therefore, a definite structural limitation is also imposed on conventional windmill performance.
For the purpose of the present disclosure, the elementary discussion given above establishes that above a certain swept diameter, the conventional windmill suffers penalties of efficiency and utility if its blade length is increased in an effort to obtain greater power recovering capability.
Winds of greater velocity obviously possess greater amounts of kinetic energy. Such winds are regularly available at higher altitudes, and access to such altitudes is granted by peaks, cliffs, and escarpments. However, such sites are also subject to seasonal extremes of turbulence, precipitation and temperature. If the rotor, weathervane pivot, and mounting tower of a conventional windmill are designed for an average condition, they are too fragile for the extreme conditions. If the rotor can withstand the extreme conditions, it is likely to be inoperative in mild conditions unless a complex feathering mechanism is provided. In short, the spectrum of useful wind velocities of the conventional windmill is very limited.
At an earlier point in time, considerable efforts were made to improve on these operational limitations of windmills. One such scheme is shown in British patent specification No. 1025 of 1908. The wind machine there disclosed has a plurality of regularly spaced outwardly extending baffles arranged around a central air driven turbine wheel. The baffles form increasingly constricted nozzles as they approach the turbine wheel thereby accelerating the air current to drive the turbine wheel. The air is then exhausted from the top of the machine.
Two problems prevent such a device from being feasibly efficient today for power conversion. Firstly, for the device to operate, a pressure differential must exist across the turbine wheel and its magnitude is directly proportional to the output power which can be produced. In this machine, the pressure differential which can exist between a side and a minor depression on the top is minimal. Secondly, only those baffle openings facing into the wind are able to collect air from the currents to accelerate toward the turbine wheel. At best, less than one-half of the blades on the turbine wheel are having positive pressure applied to them at any given time. The remainder of the turbine wheel blades are effectively acting as turbine compressor blades since they are rotating in nearly ambient pressure air. These unused blades are, therefore, wasting output torque.
A second approach is shown in U.S. Pat. No. 969,587. This wind machine houses an air driven turbine in a hollow cylindrical structure. At the opposite ends of the structure manually movable wings are employed for deflecting wind currents into the cylindrical structure to drive the turbine. The device is, however, relatively efficient when the wind is directed generally parallel to the cylinder axis. Orthogonally directed winds cannot be captured to any significant extent even by moving the deflecting wings. Furthermore, the device is not capable of self adjustment so that it cannot be left unattended for long periods of time. Also, the air flow direction through the turbine reverses direction with changes in wind direction. This necessitates reversible pitch blades on the turbine which are costly and generally fragile. Hence, this device is also wholly unsuitable for a modern energy conversion system.